beautiful: simplify metatable setting

Signed-off-by: Julien Danjou <julien@danjou.info>

lib/beautiful.lua.in

@@ -18,8 +18,6 @@ local capi =
     image = image
 }
 
-local module_name = "beautiful"
-
 --- Theme library
 module("beautiful")
 
@@ -65,9 +63,9 @@ end
 --- Get the current theme.
 -- @return The current theme table.
 function get()
-    return package.loaded[module_name]
+    return _M
 end
 
-setmetatable(package.loaded[module_name], package.loaded[module_name])
+setmetatable(_M, _M)
 
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